This invention relates generally to medical devices, and more particularly to particularly to an implantable lead.
For more than 30 years, electrical stimulation of nervous tissue has been used to control chronic pain. Therapy originates from an implanted source device, called an electric signal generator. The electrical signals, usually a series of brief duration electrical pulses, are delivered through one or more implanted leads that communicate with the source device, and contain several conductive metal electrodes to act as low impedance pathways for current to pass to tissues of interest. For example, in spinal cord stimulation (SCS) techniques, electrical stimulation is provided to precise parts of the human spinal cord through a lead that is usually deployed in the epidural space dorsal to the spinal cord. Such techniques have proven effective in treating or managing disease and chronic pain conditions.
Percutaneous leads are small diameter leads that may be inserted into the human body through a Tuohy (non-coring) needle, which includes a central lumen through which the lead is guided. Percutaneous leads are advantageous because they may be inserted into the body with a minimum of trauma to surrounding tissue. On the other hand, the designs of lead structure that may be incorporated into percutaneous leads are limited because the lead diameter or cross-section must be small enough to permit the lead to pass through the Tuohy needle, generally less than 2.0 mm diameter. Typically, the electrodes, also called contacts, on percutaneous leads are cylindrical metal structures, with a diameter of approximately 1.0 mm and a length of 4.0 to 10.0 mm. Of course, half of each of these electrodes, facing away from the tissue of interest, is not very useful in delivering therapeutic current. Thus the surface area of electrodes that face the tissue to be excited is small, typically 3.0 to 10.0 square mm. Electrodes must be approximately this size for many human applications, especially SCS, to allow sufficient charge to be delivered with each electrical pulse to excite cells, but without a high charge density (charge/pulse/square mm) that might damage tissue or the electrode itself.
Paddle leads, like Model 3596 Resume(copyright) Lead, Model 3982 SymMix(copyright) Lead or Model 3991 Transverse Tripole(copyright) Lead of Medtronic, Inc., have been developed to offer improved therapy control over some aspects of percutaneous leads. Paddle leads include a generally two-dimensional array of electrodes on one side of an insulative body, for providing electrical stimulation to excitable tissue of the body. A paddle design allows electrodes to be considerably wider than percutaneous leads, up to 4.0 mm or more. Two-dimensional arrays of electrodes allow programming of active sites and better control of the electric field distribution.
One disadvantage recognized in known paddle leads is that their installation, repositioning and removal necessitates laminectomy, which is a major back surgery done by neurosurgeons and orthopedic surgeons, involving removal of part of the vertebral bone. Laminectomy is required because paddle leads have a relatively large width (up to 1.0 cm or more) compared to percutaneous leads. Thus, implantation, repositioning or removal of a paddle lead requires a rather large opening between the vertebral bones.
Electrodes on paddle leads can easily have larger surface areas than percutaneous leads, typically 8.0 to 20.0 square mm or more. Such electrodes are mainly circular or rectangular, and require welds to fine, flexible wires passing through the length of the lead body. Such welds are prone to breakage in high flex situations unless a relatively thick paddle is used to shield the welds and support the electrodes. One advantage of preferred embodiments of the invention is that welds are not required in places where they might encounter flexing.
Because of the size and relative stiffness of paddle leads compared to the tissues they lie near to, more scar tissue or fibrosis tends to form around them over time than around percutaneous leads. This can reduce electrical efficiency, and lead to the need for larger currents over time. Such scar tissue also necessitates greater surgical efforts for removal of paddle leads, if required. On some occasions, physicians have even clipped off the lead body and left a paddle permanently in a patient rather that surgically remove it, if the system should cease giving therapeutic benefit.
For these above listed benefits and liabilities, there is a need for a lead that can be percutaneously inserted through a Tuohy-type needle, but which can create electrodes, each with substantial 2-dimensional surface area, at positions that are more lateral than the current percutaneous lead bodies. Furthermore, if such a lead could be safely removed by simple traction on the lead body, the increased surgical efforts that are required of paddle-type leads could be avoided.
The prior art has shown some examples of leads that can be expanded in situ, but they cannot perform all of the above listed features.
Mullett in U.S. Pat. No. 5,121,754 described a percutaneously-inserted epidural stimulation lead that can be straightened by a stylet and inserted into the epidural space through a Tuohy needle, and then will assume a sigmoidal shape that had been preset in it once the stylet is removed. This allows a plurality of electrodes to be positioned at a variety of longitudinal and lateral positions over the dorsal surface of the spinal cord. Because each electrode is a cylindrical metal electrode of fixed size and shape, the device cannot reliably place several electrodes at each longitudinal position. With a diameter less than 2.0 mm, each electrode must have a length of several millimeters to pass adequate currents for SCS (typically 10-20 milliamperes). Hence on such simple percutaneous leads the electrodes are manufactured as metal cylinders whose diameter matches the lead diameter. In addition, there is a problem with getting the various electrodes into lateral positions: once the stylet is removed, the preset sigmoidal shape returns, but only until the lateral forces generated by the preset curves equal the strength of various unpredictable adhesions between the dura and the vertebral bones or ligamentum flavum to resist the forces. In practice, since such leads are near the delicate spinal cord and flexible dura, they must have a high degree of flexibility once the straightening stylet is removed, and this may prevent achievement of the degree of lateral electrode positioning that is desired.
Conducting coils have been used in at least parts of leads to assist defibrillation of the heart (Smits and Camps, U.S. Pat. No. 5,105,826; Holleman, Sandstrom, Rugland and Williams, U.S. Pat. No. 4,971,070). While these have a degree of flexibility and even sigmoidal or spiraling shapes, they were designed to not change their shape, nor will they pass through a Tuohy needle lumen of 2.0 mm or less. Another conducting coil was built to have two or more alternating, generally coplanar curves to act as a defibrillation device inside the heart (Stein, U.S. Pat. No. 5,405,374). However, this has a very large curving electrode, spanning an area of approximately 40 mmxc3x9740 mm, designed to touch the heart tissue at two or three places, and does not curl back upon itself in a spiral manner. Cardiac leads often have preset curves to enable the electrodes to contact specific tissue inside the heart (Kruse, Lokhoff and van Venrooij, U.S. Pat. No. 5,628,778; Hughes, U.S. Pat. No. 4,394,866). One design had a xe2x80x9cresiliently coiled configurationxe2x80x9d, with two 360-degree turns (Ayers, U.S. Pat. No. 5,476,498), but the curving parts are insulated, several centimeters in diameter, and used for fixation of the lead inside the heart chambers.
A shape-memory neurological lead for use in the epidural space was described in WPI Acc No: 93-342955/199343. The lead as finger-shaped wings made from shape-programmable, thermal sensitive metal and/or polymer, e.g., a bimetal or nitinol alloy. At room temperature, the wings will lay along side the lead body, which can be inserted through a needle to be positioned in the epidural space. Once implanted, at body temperature the wings will move outward into their preprogrammed shape, expanding each on in one direction, to fixate the lead optimally with respect to the boundaries of the epidural space. However, there are no conducting electrodes on the tips of the stabilization wings, and the motion is more like a person raising their arms out from the body, and not, like a person with outstretched arms curling up their fingers to form fists.
Siekmeyer and van Erp (U.S. Pat. No. 5,846,196) describe a temporary multielectrode cardiac mapping probe. The probe is believed to likely have a larger diameter than will fit through the lumen of an epidural Tuohy needle (about 2.0 mm maximum). In one embodiment, two member wires are advanced out of a confining sheath inside a heart chamber, and due to their preset elastic curves, expand to stretch out a sheet array of many recording electrodes that was folded or rolled into a compact shape inside the sheath. The electrodes are each of a fixed 2-dimensional size and rectangular shape. Since the device was not intended to be permanently implanted in the human body, the advanced members are withdrawn back into the sheath after the mapping or ablation procedure is done, collapsing by rolling or folding the sheet of electrodes again to fit in the narrow width sheath. The device has the ability to carry electrodes to more lateral positions than the width of the sheath. However, the sheath must be wide enough to accommodate the widths of numerous hard, metal electrodes when the sheet of them is made compact again. If those electrodes were of the size required for tissues stimulation, and not recording electrodes, the sheath would be 10 mm in diameter or larger. The planar sheet of electrodes may have a backing of shape memory material, perhaps made of nitinol, which also can change its conformational shape due to change in temperature inside versus outside the body, or by means of heating elements.
Chilson and Smith (U.S. Pat. No. 4,699,147) also described a cardiac mapping device that had four wires each with multiple recording electrodes, that will move apart in their middle region once they are deployed out of a sheath, forming a 3-dimensional surface, but it is similar to the device in the ""196 patent, and will not perform any better for chronic tissue stimulation.
However, an optimal permanently implantable lead for tissue stimulation must have several additional features for use in the human body. It must allow the placement and use of substantially large conducting electrodes that are needed to safely and reliably pass stimulation electrical pulses of adequate amplitudes to excite tissue cells over indefinitely long periods of time, typically each about 2.0xc3x974.0 mm or larger. To greatly minimize surgical trauma during implantation, the lead should be able to have the electrodes assume a 1-dimensional shape that is very narrow (less than 0.5 mm) inside the lead body (or sheath) for passage through a small catheter or Tuohy needle, and to assume a 2-dimensional shape when outside the lead body. Since there may be considerable deposits of fibrosis or scar tissue around each electrode within a few months of permanent implantation, if necessary, the lead should be able to be removed by gentle traction on the lead body, and have all parts easily disengage from the tissue, again without major surgical trauma.
King, Rise, Schendel and Schallhom (U.S. Pat. No. 6,161,047) described seven lead designs that are compact and can be inserted through a sheath or Tuohy needle, and can be expanded in situ or even collapsed and removed through the lead body or sheath. Some of these use preset elastic materials to help the lead expand once it is in a position where expansion is safe, i.e., in a tissue space in the body. However, in each instance the conducting electrodes are metallic with a permanent, sizeable 2-dimensional surface at all times.
Furthermore, many of the current designs of implanted epidural stimulation leads do not have sufficient flexibility to function well in areas of great mechanical movement. For example, epidural stimulation leads in the cervical spinal cord are under great movement due to flexing of the neck. Percutaneous leads, and even paddle leads, can deliver paresthesia (the tingling feeling of stimulation that is necessary for pain relief). However, with currently available models, after implant the paresthesia may vary from nonexistent to very painful (too intense) during modest movements of the head. This is most frustrating to the patient, and prevents use of stimulation during sleep, when it may be most needed. Practitioners have gone to great lengths, and extensive surgery, to suture small paddles to the dura mater for cervical applications. An implantable lead with an array of electrodes that is very flexible and that even can urge each electrode toward the dura mater independently would be a very useful for epidural stimulation in the cervical region.
Finally, the dura mater is curved. Paddle leads generally are flat, so it is possible that several of their electrodes might not be touching the dura mater at all times. If some of them should be several millimeters away from the dura mater, scar tissue or even the fat cells that are found in the epidural space might become lodged between the dura mater and the electrodes, greatly diminishing the efficiency of the stimulation due to higher impedance for current that might otherwise pass into the spinal cord.
This invention relates to implantable leads for delivering electrical stimulation to tissue in the human body. Specifically, this invention relates to implantable leads that have thin, wire-like, moveable elongate members that may be elastically deformed, but with a distal tip that can curl up in a space inside the body to form a 2- or 3-dimensional electrode for delivery of electrical pulses. Members can be positioned axially or at variable non-axial distances from the lead body. This invention also relates to mechanisms for accomplishing the insertion of multiple electrodes in a manner that is minimally invasive, even through a narrow lumen like a vertebral foramen. An array of such electrodes can also be easily removed without major surgical intervention.
Preferred embodiments of the invention combine the advantages of percutaneous leads with those of paddle leads, both of which are permanently implanted in the human body for electrical stimulation of excitable tissue. In a preferred embodiment, a lead body is provided that can be passed through a Tuohy needle and which can spread over several dimensions an array of 2-dimensional electrodes. These electrodes are located on the tips of moveable, extendable members, which, once deployed beyond the confines of the lead body, will curl up into 2-dimensional electrodes. If the lead should need to be removed, the lead body or its extendable members can be retracted, and the electrodes will uncurl and become straight as they are drawn back into the lead body. This can be done without major surgical intervention.
The part of an extendable member that curls into a 2-dimensional conductive pad or 3-dimensional electrode is composed of a robust and safe material, such as platinum or platinum/iridium. Those metals, or a composite of similar metals over a substrate, are treated by heat, pressure or chemicals so that they have a preset tendency to curl up, especially when it is no longer confined in a channel of the lead body. The tip that curls may be a coiled conductor, much like a spring.
In an embodiment, the curling part of an extendable member may have a bimetallic nature so it will curl at a given temperature, or it may be made of nitinol or other hyperelastic materials, that may require heating to certain temperatures to effect shape changes.
Regarding the positioning of electrodes, in a preferred embodiment, each extendable member can be positioned independently, or groups of them can be moved in unison. Each member has a portion that may have a preset curve to allow the tip of that member, with its curled electrode, to be positioned more laterally or more ventrally (toward the dura matter for epidural stimulation) than the tip of the lead body itself. Each member may have an asymmetry to match an asymmetry in its channel, so that its deployment is in a fixed direction from the lead body. Alternatively, the implanting physician may be able to use fluoroscopy to send each member""s tip in any preferred direction.
By having a curve to allow deployment of the extendable member""s tip non-axially, various degrees of non-axial placement of a electrode can be controlled by the length of deployment of the extendable member outside of the lead body. The member in this case needs an elastic ability to be straightened (when so confined) or to curve (when no longer confined).
For epidural SCS, if there is a curve in the extendable member to allow deployment of the member""s tip ventrally (toward to the spinal cord), each member may be positioned to allow it""s curled top to lie against the curved surface of the dura matter. Thus, an array of such electrodes can match the curvature of the dura mater, and keep a more constant distance from the spinal cord.
In an embodiment, the extendable member may be composed of a coiled conductor to have great flexibility. This design would use an internal wire spanning at least some portions to give the member sufficient curvature to allow its deployment from the lead body in specific directions. There would be insulation on the outside except at the proximal end, which is electrically connected to the pulse source, and at the distal end, which curls into a conducting electrode. There might also be two or more coiled conductors, dissimilar in properties, which are bonded, hooked or welded to the tip of the member.
In another embodiment, a coiled conductor may be found only at the distal end of the extendable member, with most of the length of the member being a simple metal wire, insulated to prevent current loss except at the conducting tip. This coiled conductor may screw on to the end of the wire.
In order to prevent curling of the electrode before the end of the extendable member is in its final position, the tip of the member may be coated with a material that keeps it rigidly straight. This material would dissolve over time in the environment of the body, allowing curling of the tip into an electrode. The material may also have a sharp point, to make the deployment of the member through adhesions or other tissue easier.
In a further embodiment, the lead may be designed to allow placement of sizeable 2-or 3-dimensional electrodes through a very small lumen in the body, such as a vertebral or sacral foramen, for peripheral nerve stimulation. This can be done with a smaller diameter lead body than other currently available lead designs, which have rigid electrodes.
Other advantages, novel features, and further scope of applicability of the present invention will be set forth in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. For example, although the examples herein depict electrical electrodes that are essentially 2-dimensional, a 3-dimensional ball electrode may also be assembled by curling of the tip of an extendable member that has been appropriately preset by treatments.